This invention relates to a nonadjusting battery life detector which detects the battery life by detecting the voltage drop thereof.
A conventional nonadjusting battery life detector is shown in FIG. 1. A constant current source 3 is connected to a constant voltage source 2 and the output terminal thereof is connected to the reference voltage input terminal V.sup.+ of a differential amplifier 1. Therefore, a reference voltage having a particular voltage relative to ground G is supplied to the input terminal V.sup.+. On the other hand a battery voltage is divided by resistors R.sub.1 and R.sub.2 and a divided voltage is supplied to a comparison voltage input terminal V.sup.- of the differential amplifier 1 which detects the voltage drop of the battery. The variation .delta.V.sub.D of the battery voltage is also divided by the resistors R.sub.1 and R.sub.2 and this divided variation is supplied to the comparison voltage input terminal V.sup.- of the differential amplifier 1, too. The variation of the detecting voltage for the battery life is determined by the sum of the first variation of the output voltage of the constant voltage source 2, the second variation from a design value of the voltage supplied to the reference voltage input terminal V.sup.+ of the differential amplifier 1 and the third variation from the design value of the comparison input voltage to the comparison voltage input terminal V.sup.-. Now, the second variation occurs because of the offset voltage of the differential amplifier 1. The variation of the detecting voltage for battery life is obtained by multiplying the variation from design value of the comparison input voltage by (R.sub.1 +R.sub.2)/R.sub.2.
Furthermore, when the nonadjusting battery life detector is used in a wrist watch having an integrated circuit which operates with low power, it is necessary to operate the nonadjusting battery life detector in a sampling operating mode in order to reduce the power consumption thereof. In the case that the sampling operating mode is used, a total variation is obtained by adding the variation from the design value of the voltage for detecting the battery life to the variation of the voltage drop (V.sub.CE sat) between the emitter and collector of transistor T.sub.r1. For example, when the output voltage of the silver oxide battery drops from 1.57 volts to 1.4 volts, an output of the differential amplifier 1 having a battery life detector is designed so as to be inverted. In this case, the variation of detector voltage is about .+-.50 mV, that is the detector voltage is within 1.4 V.+-.50 mV.
It is an object of present invention to remove the defect described above, that is, in the case that the nonadjusting battery life detector is operated in the sampling operating modes the variation from the design value of the detecting voltage for battery life is within .+-.25 mV.